An integrated circuit microcontroller can be used to generate the signals that power electric motors, such as alternating current (AC) induction motors. The drive signals are typically generated from a direct current (DC) power supply that is switched and alternately inverted. One switching technique used to generate the drive signals is called space-vector pulse width modulation (PWM). For example, in an electric motor with three induction coils each separated by 120 degrees, space-vector PWM governs the times and the durations at which six switches are opened and closed to produce the six pulsed signals for the inverter stage of the motor drive. Space-vector PWM is used not only to generate the drive currents, but also to control the electric motor. Space-vector control defines the PWM time intervals for each of the six sectors of the electric motor to control the rotation of the electric motor.
In order to perform space-vector control, however, the microcontroller should accurately determine the currents and voltages emitted from the inductive coils of the electric motor. As the drive currents into the coils increase and decrease, expanding and collapsing magnetic fields are created that in turn generate currents in addition to the drive currents. The sum of these currents can result in a voltage drop across a voltage sensor that falls below the ground potential of the DC power supply. Therefore, a voltage below ground potential should be measured in order accurately to determine the currents and voltages emitted from the inductive coils.
Various circuits have been used to sense voltage for space-vector control of AC induction motors. Some voltage sensing circuits require a negative voltage supply. But it is expensive to add to the circuit a second power supply that is below ground potential. Other voltage sensing circuits employ two differential pair of transistors. These sensing circuits typically sense negative voltages only down to one threshold voltage below ground potential. Moreover, sensor interface amplifiers employing two differential pair of transistors typically do not generate a linear amplification of input voltages at the low end of their response ranges.
A sensing circuit is sought that senses voltages more than one threshold voltage below ground potential and that does not use a below-ground power supply.